1. Field
The disclosure relates to negative active materials, methods of preparing the same, negative electrodes for lithium secondary batteries that include the negative active materials, and lithium secondary batteries including the negative electrode.
2. Description of the Related Art
Lithium secondary batteries used in portable electronic devices, such as personal digital assistances (PDAs), mobile phones, notebook computers; electric bicycles; electronic vehicles, etc. have discharge voltages that are at least two times greater than that of conventional batteries. Accordingly, lithium secondary batteries exhibit high energy densities.
A typical lithium secondary battery includes an electrode assembly including a positive electrode, a negative electrode, and either an organic electrolytic solution or a polymer electrolyte filling the space between the positive and negative electrodes. The positive and negative electrodes each include an active material that allows lithium ions to be intercalated and deintercalated. In this structure, when lithium ions are intercalated and deintercalated between the positive and negative electrodes, oxidation and reduction reactions occur, and thus, electrical energy is generated.
The positive active materials of lithium secondary batteries may be oxides of lithium and a transition metal that allow intercalation of lithium ions, such as a lithium cobalt oxide (LiCoO2), a lithium nickel oxide (LiNiO2), or a lithium nickel cobalt manganese oxide (e.g., Li[NiCoMn]O2 or Li[Ni1-x-yCoxMy]O2).
Research into negative active materials that allow intercalation and deintercalation of lithium ions, such as various types of carbonaceous materials including artificial and natural graphite and hard carbon, and non-carbonaceous materials such as Si, has been conducted. However, non-carbonaceous materials such as Si repeatedly undergo volumetric expansion and contraction during intercalation and deintercalation of lithium ions, and thus, a negative electrode including such a non-carbonaceous material has an unstable structure and decreased cycle-life. To address problems with carbonaceous and non-carbonaceous active materials, research into Si-based alloys has been conducted.
Si-based alloys may be, for example, Si—Ti—Ni alloys. A Si—Ti—Ni alloy comprises a matrix phase of Si7Ti4Ni4 in the alloy where an atom percent (at %) ratio of Si to Ti to Ni is 7:4:4. However, this matrix phase contains a relatively large amount of Si, and thus a large amount of Si in the alloy is consumed in an inactive phase that does not react with lithium. In addition, raw material costs for Si—Ti—Ni alloys are high.